Endurance of ill-fitting red outfit (9)
I believe the answer is:
fortitude
'endurance' is the definition.
(thesaurus)
'ill-fitting red outfit' is the wordplay.
'ill-fitting' indicates an anagram.
'red'+'outfit'='redoutfit'
'redoutfit' anagrammed gives 'FORTITUDE'.
'of' is the link.
(Other definitions for fortitude that I've seen before include "Courage in adversity" , "Stiff upper lip" , "Mental and emotional strength" , "Conceited (informal)" , "Bravery" .)